Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes.

A new air-stable catalyst for the oxidative dehydrogenation of benzylic alcohols under ambient conditions has been developed. The synthesis and characterization of this compound and the related monomeric and dimeric V(IV)- and V(V)-pin (pin = perfluoropinacolate) complexes are reported herein. Monomeric V(IV) complex (MeN)[V(O)(pin)] () and dimeric (μ-O)-bridged V(V) complex (MeN)[V(O)(μ-O)(pin)] () are prepared in water under ambient conditions. Monomeric V(V) complex (MeN)[V(O)(pin)] () may be generated via chemical oxidation of under an inert atmosphere, but dimerizes to upon exposure to air. Complexes and display a perfectly reversible V couple at 20 mV (vs Ag/AgNO), whereas a quasi-reversible V couple at -865 mV is found for . Stoichiometric reactions of with both fluorenol and TEMPOH result in the formation of (MeN)[V(O)(μ-OH)(pin)] (), which contains two V(IV) centers that display antiferromagnetic coupling. In order to structurally characterize the dinuclear anion of , {K(18C6)} countercations were employed, which formed stabilizing K···O interactions between the counterion and each terminal oxo moiety and H-bonding between the oxygen atoms of the crown ether and μ-OH bridges of the dimer, resulting in {K(18C6)}[V(O)(μ-OH)(pin)] (). The formal storage of H in is reversible and proton-coupled electron transfer (PCET) from crystals of regenerates upon exposure to air over the course of several days. Furthermore, the reaction of (2%) under ambient conditions with excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in the selective formation of fluorenone (82% conversion), cinnamaldehyde (40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol dehydrogenation (OAD) chemistry known for VO surfaces and demonstrating, in air, the thermodynamically challenging selective oxidation of alcohols to aldehydes/ketones.